How to balance the charging of the protection board of the lithium battery pack?

by:CTECHi     2021-08-10

Now is the age of electrical appliances. In our daily life, we need to charge everywhere and all the time. To charge, we must have a lithium battery (rechargeable) and a lithium battery protection board to protect the lithium battery. Now we come Talking about when the power lithium battery is used in groups, each lithium battery requires the protection of charging over-voltage, discharge under-voltage, over-current, and short-circuit. During the charging process, the problem of balanced charging of the entire group of batteries is designed. A battery pack protection board with equalizing charging function that protects any number of groups of lithium batteries connected in series. Simulation results and industrial production applications prove that the protection board has perfect protection functions, stable operation, high cost performance, and equalization charging error less than 50mV. Let's think about it, the lithium battery protection board has such a big function, and how does it make the lithium battery protection board equal charge? Today, let’s learn from the people’s creation and development.

When charging a group of lithium batteries in series, ensure that each battery is charged in a balanced manner, otherwise the performance and life of the entire group of batteries will be affected during use. Commonly used equalization charging techniques include constant shunt resistance equalization charging, on-off shunt resistance equalization charging, average battery voltage equalization charging, switched capacitor equalization charging, step-down converter equalization charging, inductance equalization charging, etc. However, the existing single-cell lithium battery protection chips do not have a balanced charge control function; the balanced charge control function of a multi-cell lithium battery protection chip requires an external CPU, which is realized through serial communication with the protection chip (such as I2C bus). The complexity and design difficulty of the protection circuit are reduced, the efficiency and reliability of the system are reduced, and the power consumption is increased.

Basic working principle of equalizing charging of lithium battery pack protection board

The schematic diagram of a lithium battery pack protection board with equalizing charging capability designed with a single-cell lithium battery protection chip is shown in the figure: 1 It is a single-cell lithium-ion battery; 2 is the charge overvoltage shunt discharge branch resistance; 3 is the switching device for shunt discharge branch control; 4 is the overcurrent detection protection resistance; 5 is the omitted lithium battery protection chip and circuit connection part; 6 is a single-cell lithium battery protection chip (generally including charge control pin CO, discharge control pin DO, discharge overcurrent and short circuit detection pin VM, battery positive terminal VDD, battery negative terminal VSS, etc.); 7 is charging overvoltage The protection signal is isolated by the optocoupler to form a parallel relationship to drive the gate of the charge control MOS tube in the main circuit; 8 is the discharge undervoltage, overcurrent, and short circuit protection signal after the optocoupler is isolated to form a series relationship to drive the MOS for discharge control in the main circuit Tube grid; 9 is a charge control switch device; 10 is a discharge control switch device; 11 is a control circuit; 12 is a main circuit; 13 is a shunt discharge branch. The number of single-cell lithium battery protection chips is determined according to the number of lithium battery cells, and they are used in series to protect the corresponding single-cell lithium battery from charging and discharging, overcurrent, and short-circuit conditions. While charging and protecting, the system realizes balanced charging through the protection chip to control the on and off of the shunt discharge branch switch device. This solution is different from the traditional method of achieving balanced charging at the charger end, and reduces the design of the lithium battery pack charger. The cost of the application.

When the lithium battery pack is charged, the positive and negative poles of the external power supply are connected to the positive and negative poles of the battery pack, BAT+ and BAT- respectively, and the charging current flows through the positive and negative poles of the battery pack, BAT+ and the battery Single-cell lithium battery 1~N, discharge control switch device, charge control switch device, battery pack negative pole BAT- in the group, the current flow is as shown in the figure

The control circuit part of the system is single The charging over-voltage protection control signal of the lithium battery protection chip is output in parallel after being isolated by an optocoupler to provide the gate voltage for the conduction of the charging switch device in the main circuit; for example, a certain or several lithium batteries enter first during the charging process In the over-voltage protection state, the shunt discharge branch connected in parallel to the positive and negative ends of the single-cell lithium battery is controlled by the over-voltage protection signal to discharge, and at the same time the corresponding single lithium battery connected in series in the charging circuit is disconnected from the charging circuit.

When lithium battery packs are charged in series, the impact of the difference in single cell capacity is ignored. Generally, the battery with lower internal resistance is charged first. At this time, the corresponding overvoltage protection signal controls the switching device of the shunt discharge branch to close, and a shunt resistor is connected in parallel at both ends of the primary battery. According to the PNGV equivalent circuit model of the battery, at this time, the shunt branch resistance is equivalent to the load of a single-cell lithium battery that is charged first, and the battery discharges to maintain the battery terminal voltage in a very small range near the fully charged state. Assuming that the first lithium battery is charged first and enters the over-voltage protection state, the current flow in the main circuit and the shunt discharge branch is shown in Figure 3. When all single-cell batteries are charged and enter the over-voltage protection state, the voltages of all single-cell lithium batteries are completely equal within the error range, and the charge protection control signal of each protection chip becomes low, and the charge control switch device in the main circuit cannot be Provide the grid bias voltage to turn it off, and the main circuit is disconnected, that is, equalizing charging is realized, and the charging process is completed.

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